Pursuant to 35 USC xc2xa7xc2xa7371 and 365(b) the priority of Application Nos. PCT/EP 99/03897 filed Jun. 5, 1999 and DE 198 27 4734 filed Jun. 19, 1998 is claimed.
Cross-flow filtration is generally performed with cassette filtration systems wherein several filter cassettes are arranged sequentially as a unit, being pressed between mounting plates that are sealed with sealants in their peripheral edge regions so as to be fluid-tight. The mounting plates are constructed as leading or trailing end plates with corresponding ports and distributors in channels for feed intake, retentate discharge and permeate discharge. See, for example, WO 96/28240 and EP 0 345 209. If the sealants in the peripheral edge regions involve hard-setting compounds such as polypropylene, the filter cassettes are rendered fluid-tight by using elastomeric sealants. With permanently elastic sealing compounds such as silicone, fluid-tight mounting is achieved without the need for additional sealants. In cross-flow filtration, the fluid to be filtered is fed through the leading end plate and the corresponding channels into the feed channel of the filter cassettes for the fluid to be filtered. It flows along the permselective membrane surfaces and exits the cassette as retentate. Some of it passes through the permselective membranes of the cassette and is discharged from the system through corresponding channels and the trailing end plate as permeate. Fluid flows and transmembrane and internal pressures are regulated by pumps and valves. For optimal operation of crossflow filtration systems of this kind, pumps must be adjusted to the configuration of the filter cassettes and in particular to the size of the feed channel opening. Transmembrane pressures and fluxes required for efficient filtration must not be reached or exceeded.
Cross-flow filter cassettes are known and disclosed in, for example, U.S. Pat. No. 4,715,955 and DE 34 41 349. Such cassettes are constructed of a multiplicity of adjacent filter cells, each cell consisting of alternating flat sheet membranes straddled by flat screen members, retentate spacers to form flow channels for the fluid to be filtered, and filtrate spacers to form filtrate-collection channels. Screen members and membranes have axially aligned holes, preferably running perpendicular to their surfaces, to form channels for fluid feed intake, retentate discharge and filtrate discharge.
To protect the membranes from potential mechanical damage in the transition region of the sealant potentially arising from the membrane being pressed on or into the retentate and filtrate spacers too firmly, textile-reinforced membranes are often used, wherein which one or both membrane surfaces are covered by a textile reinforcement such as a fiber fleece. However, such textile-reinforced membranes generally have a reduced flux relative to non-reinforced membranes, thereby lowering their filtering capacity. As an alternative, DE 34 41 249 recommends that additional protective ring masks and protective frames be included between the screen members. In addition to protection of the non-reinforced membranes from mechanical damage achieved thereby, this causes the flow channel for the fluid to be filtered to be expanded in size, forming a so-called xe2x80x9cwide-channelxe2x80x9d module. By varying the thickness of the protective ring masks and protective frames, the size of the flow channel can be set within certain limits. While the provision of such ring masks and frames provides a number of advantages such as good strength, lower mechanical loads on the membranes, and the ability to filter viscous media, at the same time it has the disadvantage that with the expanded flow channel, the flow-through rates required for optimal filtration of the fluid to be filtered through the membrane surfaces can be achieved only by exceptionally high fluxes. As a consequence systems equipped with filter cassettes of this kind have a high energy demand. Moreover, such cassettes do not achieve optimal performance in systems equipped with lower pumping capacity.
The use of filter cassettes that have no expanded flow channel such as in a so-called xe2x80x9cnarrow-channelxe2x80x9d module, does have the advantage that high fluxes can be readily achieved with low pumping capacity, but at the same time has the disadvantages of being restricted to the use of thinner spacers with consequent higher mechanical loads on the membranes and reduced membrane surface area available for filtration. By using finer fabrics as spacers, the number of non-filtering points where the reinforcing fabric fibers are applied to the membrane is increased in comparison to thick, coarse fabrics or filter cassettes with built-in protective ring masks and protective frames. In addition, filter cassettes of this type exhibit low permeability to particles and poor filtration of viscous media.
It is therefore a principal object of the present invention to provide improved filter cassettes that can be operated in cross-flow filtration systems having a wide variety of pumping capacities. A related object of the invention is the provision of improved cross-flow filter cassettes that can be operated even when equipped with non-reinforced membranes as in the case of a narrow-channel module.
The cross-flow filter cassettes according to the present invention are characterized by the fact that they have at least one retentate spacer in which the inlets to the open holes forming one type of channel such as a retentate discharge channel are larger than the inlets to the open holes forming a second type of channel such as the feed inlet channel. If the cross-flow filter cassettes are connected to the leading and/or trailing end plates in such a way that greater access to the open holes of the retentate spacers is formed by the channels for the fluid flow, a pressure drop arises in the other longitudinal flow channels which have equal access to the open holes to the channels for the fluid intake and retentate outflow. Due to this higher pressure level in a given longitudinal flow channel across the retentate spacer, that longitudinal flow channel expands and exerts a force on adjacent flat sheet membranes, which in turn expands other longitudinal flow channels, thereby increasing flux for a given feed pressure. Alternatively with equal fluxes, this leads to a lower amount of fluid feed having to be pumped through the filter cassette per unit of time.
In an alternative embodiment of the invention, the membranes are covered in their edge regions by protective frames that leave the holes open or in the region of their holes by protective ring masks that leave the holes open. This permits mechanically fragile non-reinforced membranes to be installed in the filter cassettes, but at the same time it leads to expansion of the longitudinal flow channels, requiring greater pumping capacity, in order to achieve optimal flow through the membranes. But this negative effect is overcome by the invention whereby the longitudinal flow channels flow are narrowed.
With the cross-flow filter cassettes according to the invention, fluids are filtered such as liquids, emulsions, suspensions, foods, and drinks such as beer, beer seasonings, wine, juice, water, mineral water, and milk; drinking, process, and waste water; and solutions in pharmaceuticals, medicines, cosmetics, chemistry, biotechnology, gene technology, electronics, environmental protection, and laboratories. They can be used to separate materials, to disinfect and sterilize solutions, and to remove pollutants from fluids, for filtration and concentration of biological solutions, to separate microorganisms such as bacteria, yeasts, viruses, and cell components, for desalinization of protein solutions and other biological media, and for separating materials from ions, macromolecules, and biological molecules.